A finder optical system of an SLR camera has a configuration in which an object image passes through a photographing lens, is reflected by a quick-return mirror, is formed as a right-to-left inverted image on a focusing plate, is erected by an image-erection member such as a penta roof prism (pentagonal roof prism), and is observed with a loupe optical system. In order to facilitate observation of the object image, there is a demand for a bright finder optical system that has a large observation magnification. It is known that if the refractive index of the image-erection member is increased in order to increase the observation magnification, since the air-equivalent length (geometric thickness along the optical axis/refractive index) of the image-erection member can be shortened, the focal length of the loupe optical system can also be shortened and the observation magnification can be increased.
On the other hand, generally speaking, the higher the refractive index of the material, the lower the transmittance at the short wavelength side from around the vicinity of 450 nm to 460 nm compared to the transmittance at the long wavelength region. Since the geometrical thickness of the image-erection member is approximately 70 mm through 100 mm, which is a large thickness, if a material having a high refractive index is used for the image-erection member, although the observation magnification can be increased, the transmittance of the light bundle deteriorates at the short wavelength region, so that the finder image appears visibly yellow.
In Patent Literature 1, an example is disclosed in which a material having a high refractive index (1.69680 or 1.73400) is used for an image-erection member (penta roof prism). However, there is no reference whatsoever in Patent Literature 1 in regard to the problem of coloration of a finder image, nor in regard to countermeasures therefor.
Patent Literature 2 discloses a reflection-peak wavelength of spectral reflection characteristics for a first reflection member (the reflection member at the emitting side), out of a plurality of reflection members of the image-erection member, being set to within a range of 380 nm through 500 nm in order to prevent a coloration phenomenon caused by the image-erection member (penta roof prism); and discloses, as a more preferable embodiment, a reflection-peak wavelength of spectral reflection characteristics for a second reflection member (the reflection member at the incident side) being set to within a range of 500 nm through 600 nm. However, in Patent Literature 2, the image-erection member only has a refractive index of approximately 1.56, and therefore having a problem with the observation magnification being insufficient. Furthermore, although Patent Literature 2 pays attention to “a coloration phenomenon caused by the image-erection member”, the problems that occur if the observation magnification is attempted to be further increased are not sufficiently solved.